The increase in the rated power of wind turbines entails the increase in weights and dimensions of all the turbine components in general, but are of special relevance for the assembly costs of the following aspects:                The height of the tower;        Diameter of the rotor and weight of the blade-axle housing unit;        Weight of the nacelle and subcomponents;        Weight of the tower sections.        
Of the above, the most relevant is the height of the tower, of which there are already designs of 120 m and more, especially for the assembly of relatively voluminous and heavy components such as the aforementioned: the large overturning moments associated to the increase in height make it necessary to have a high capacity crane (much greater than that simply associated to the weight of the components).
The use of these cranes is expensive, firstly due to the problems of availability that make the daily rental cost expensive: there is not a large amount of this type of cranes; and second, the high costs associated to transport due to the large number of trucks that are needed to move them. According to data shown in U.S. Pat. No. 8,011,098 B2, the rental costs of the crane for tower assembly, can come to $80000 per week, together with the almost $100000 of transport costs (using forty or more trucks).
These high costs justify the search for alternative means to build wind turbines, including the tower.
References are known in the state of the art related to two types of solution:                Self-climbing systems (understanding as such structures that are supported and raised throughout the tower);        Bridge-crane type solutions supported on lattices on the ground.        
Among the climbing solutions, patent U.S. Pat. No. 6,868,646 B2 is known regarding a method and means for erecting a wind turbine tower.
Said means incorporate two structures: a lower one, whereto a hoist cable is fastened in a point close to the lower part, and an upper one, which supports the bridge crane-type structure. The hoist cable passes through a pulley fastened to a suitable fastening point in the tower close to the upper part of each section and it is collected in a winch located on the ground.
The system goes up and down the tower every time a section is assembled. Then, once assembled above the lower sections of the tower, the system descends, another section is again fastened and it is again hoisted. The lower structure incorporates a wheel/roller system adapted to cooperate with guides situated in the tower.
International application WO 2011/031577 A2 relates to a method and apparatus for wind turbine erection incorporating a caterpillar type system secured to the hoisting structure which makes it possible to move the hoisting structure along the tower by the effect of friction force between the tower and the caterpillar type system. Later, once the hoisting structure is in position, it is fastened to the tower and a new section is assembled. Therefore, the invention discloses caterpillar type systems which adapt to the conical shape of the tower. This system may be expensive due to its complexity, especially in conical solutions.
In terms of bridge crane-types structures, patent U.S. Pat. No. 8,011,098 B2 is known which discloses a crane-type structure supported by a self-supporting lattice incorporating actuators capable of lifting the lattice structure and the crane above them for inserting sections of additional lattice and therefore, gain height to be able to assemble more sections of the tower as it grows in height, and lastly the nacelle and the rotor. With the current tower perimeter dimensions, the sections of lattice must be assembled in situ, since they would exceed the transport dimensions. In said structure the tower sections are horizontally fed by means of carriages to also avoid the need of dispensing the sections by means of auxiliary cranes that position them close to the tower base.
The self-climbing structures can be light and of reduced dimensions, as they use the tower as support of the weight of the components assembled (or as much as it is assembled therefrom).
However, the self-climbing structures by pulleys, jacks or similar require a type of structure guide on the tower to guarantee that there is no interference with the tower during the lifting of the structure. In the case of a conical tower, a typical design if concrete is used, the guiding system becomes complicated, as it requires a spring-type element which guarantees the compression of wheels or rollers on the tower surface and which avoids said interference. Alternatively, the climbing is performed by friction between a mechanism disposed on the structure and the tower. In this case, it again requires that the mechanism is capable of adapting to the tower's conical nature.
Furthermore, the lattice-based structures, such as that shown, are large structures that will require a large number of trucks for their transport. They have the advantages, compared with climbing solutions, in that they can be used with any type of tower without the need to vary their design, since it is not necessary to provide them with additional fastening points or supports to fasten the climbing structure.